Field of the Invention
The present invention relates to resolution conversion processing for outputting a high-resolution image signal by a low-resolution image forming apparatus.
Description of the Related Art
In general, in image processing in an image forming apparatus, such as a printer, a copying machine, and an MFP, PDL (Page Description Language) data is developed into bitmap data within a controller of the apparatus. Then, the bitmap data is output with the same resolution as that of the developed data. For example, data that is developed with a resolution of 600 dpi is output by a printer having a printing resolution of 600 dpi.
On the other hand, a configuration in which PDL data is developed into bitmap data and then output with a resolution higher than the printing resolution of an image forming apparatus has been proposed. In this configuration, for example, PDL data is developed into 1,200-dpi bitmap data and each pixel in the developed data is output by a 600-dpi printer by using the spot multiplexing technique. With the technique to output a high-resolution image signal as high-definition image data by a low-resolution image forming apparatus, it is possible even for a printer having a low printing resolution to provide printed matter of high image quality (see Japanese Patent Laid-Open No. 2004-201283).
For example, in the technique according to Japanese Patent Laid-Open No. 2004-201283, in the case where resolution conversion is performed to convert first image data having a first resolution M into second image data having a second resolution N lower than the first resolution M, a conversion filter is used. For example, it is assumed that the resolution M is 1,200 dpi and the resolution N is 600 dpi. In this case, resolution conversion processing is performed by using a conversion filter in which a pixel of interest is arranged at every two pixels in the main and sub scan directions and a 1,200-dpi binary image is converted into a 600-dpi multivalued image. At the time of conversion, the technique according to Japanese Patent Laid-Open No. 2004-201283 does not take into consideration the position relationship between the pixel of interest in the conversion filter and the dither growth center position. Because of this, there is a case where the dot growth center point deviates from the sampling point at the time of performing resolution conversion processing on the dot growth screen. In the case where the dot growth center point deviates from the sampling point at the time of resolution conversion processing, there is a possibility that the smoothness of the change in color (gradation properties) will be lost.